The present invention relates generally to an optical differential tilt sensor, and more particularly to a high bandwidth differential tilt sensor for use in the control of phased array telescopes.
In the past the use of multiple optical telescopes in a phased array has encountered a number of operational problems. One of these problems has been experimentally observed to involve low frequency drift in the relative positions of the images from each of the individual telescopes. In order to keep the telescopes properly pointed and phased with respect to each other, frequent manual compensation had to be provided. Furthermore, this manual compensation had to be provided on a continuous basis, since the environment continually disturbs the optical, mechanical and electronic elements of the telescope system.
U.S. patents of interest include No. 4,639,586 to Fender et al, which teaches a technique using samples of the transmitted beam to control optical path lengths and therefore the phase through separate telescopes so that the beams add coherently at the receiver. Carreras et al in U.S. Pat. No. 4,667,090 teaches a plurality of telescopes wherein output return signals are applied directly after sampling to a photo-detector for determination of the constant and transient tilt error of the telescope system.
The drift problem in multi-telescope trackers has been addressed by Carreras et al. The Carreras invention solves the problem by augmenting the standard wide bandwidth tilt sensor which measures the tilt error in each telescope with a low frequency differential tilt sensor which is common to all three telescopes. The low frequency sensor is immune to the drift problem since it uses a common quad cell photo-detector, common optics, and largely common electronics for all three telescopes. It measures only tilt differences among the three beams, and therefore is insensitive to any low frequency drift that affects all three beams in the same way.
The main problem with the Carreras invention is the use of the mechanical chopper. The chopper produces vibration and air turbulence which decrease the overall accuracy of the tracker system. The chopper makes the optical system more complicated since it does not eliminate the need for the standard wide bandwidth tilt sensor.